True, but it would be worse with a metal fitting. If you used an NPS fitting, the hoop stress would be lower, since it isn't tapered. No need to use a threaded nipple if you're epoxying it in place though.
SD, I agree with you but I am concerned that it would be very difficult to get epoxy to adhere well enough to the Polypro to glue a nipple in place. PP is just too slippery to glue easily.
Another idea. Find a polypropylene nipple, threaded on one end only. Cut off the old nipple, clean the stub of the old nipple and the unthreaded end of the nipple well. Chuck the nipple up in a drill (preferably a drill press, or make sure you have very steady hands and great alignment). Put the drill on high speed and spin weld the new nipple onto the fixture. Forward Technology | Plastic Assembly - Spin Welders
This is really done best with a drill press and the fixture well supported, you spin the nipple while apply light pressure of it against the fixture. When the plastic starts to melt you stop the spinning and maintain pressure until the plastic hardens. It does result in pretty solid bonds.
) has a decent selection of polypropylene pipe nipples.